This invention relates to a method and apparatus for detecting liquids in a container in which all electrical conductors may be placed exterior to the container.
Conventional methods of detecting the presence of fuel in aircraft fuel tanks involve placement of capacitive sensors within the tanks. When fuel surrounds the sensors, their capacitance changes to indicate that a certain fuel level (the level of placement of the sensors) has been reached. Of course, electrical conductors extend into the fuel tank to connect to the capacitive sensors.
The above-described method presented little safety problems when aircraft fuel tanks were carried within all metal wings since the wings would act as electrostatic shields to prevent lightning, for example, from reaching the interior of the fuel tanks. However, with the advent of the use or proposed use of non-metallic material, such as graphite, in wing construction, the "electrostatic shield" feature was lost. With this construction, lightning might penetrate the non-metallic material to reach electrical conductors of fuel measuring circuitry and the electric charge of such lightning could thus be conducted to the interior of the fuel container to cause an explosion.
In addition to the safety problem above discussed, existing fuel sensing systems are difficult to service and repair, at least as to that portion which is located within the fuel tank. This difficulty could be reduced by placing as much of the fuel sensing system as possible exterior to the tank.